WordPress at a glance
filter is not described

attachment_fields_to_save filter-hook . WP 2.5.0

Filters the attachment fields to be saved.

Usage

add_filter( 'attachment_fields_to_save', 'filter_function_name_5134', 10, 2 );
function filter_function_name_5134( $post, $attachment ){
	// filter...

	return $post;
}
$post(array)
An array of post data.
$attachment(array)
An array of attachment metadata.

Where the hook is called

media_upload_form_handler()
edit_post()
wp_ajax_save_attachment_compat()
wp-admin/includes/media.php 683
$post = apply_filters( 'attachment_fields_to_save', $post, $attachment );
wp-admin/includes/post.php 324
$post_data = apply_filters( 'attachment_fields_to_save', $post_data, $attachment_data );
wp-admin/includes/ajax-actions.php 2589
$post = apply_filters( 'attachment_fields_to_save', $post, $attachment_data );

Where the hook is used (in WP core)

wp-admin/includes/admin-filters.php 33
add_filter( 'attachment_fields_to_save', 'image_attachment_fields_to_save', 10, 2 );